Preheat the oven to 375°F, and prep a 9x13" baking dish by spraying it with cooking spray.
Cook the noodles following package instructions, then drain them and set them aside. Add a little butter or oil to make sure they don't start sticking.
12 oz. egg noodles
Saute the onion. Add half the butter to a large skillet alongside the finely diced onion, and cook on medium-high heat until the onions are translucent.
1 small onion, 8 Tbsp. butter
Add the flour and whisk until a thick paste forms. Let it bubble for 3-4 minutes, until it smells richly buttery and toasted, then slowly pour in the broth and milk, whisking continuously until smooth and creamy.
4 Tbsp. flour, 2 cups whole milk, 1 cup chicken broth
Add in the rest of the ingredients, starting with the nutmeg and Italian seasoning, then the mushrooms, turkey, peas, and both kinds of cheese. Season with salt and pepper to taste. Then add in the egg noodles, stirring gently to combine. Don't over-mix, you can easily break the noodles!
1 tsp. italian seasoning, ¼ tsp. nutmeg, 3 cups turkey meat, 6 oz. canned sliced mushrooms, ¾ cup frozen peas, ½ cup parmesan cheese, ¼ cup mozzerella cheese, salt and pepper
Pour the mixture into the prepared casserole dish and smooth it out into an even layer.
Melt the remaining butter in a small pan or pot and add in the panko breadcrumbs. Stir until buttery and toasted, about 3 minutes. Then remove from the heat and top the casserole with the crumbs.
1 cup panko breadcrumbs
Bake the casserole for 30 minutes, uncovered, until bubbly and golden. Serve, and enjoy!